The present invention relates to a solid-state image pickup device, and more particularly to the structure of a photoelectric cell in a solid-state image pickup device.
Solid-state image pickup or camera devices have been conventionally used in various fields of facsimiles or OCRs (Optical Character Readers) etc. Since solid-state image pickup devices having a large number of light sensitive pixels can be made up, particularly in recent years, use as the substitution for pickup tubes for cameras is being increased.
A conventional solid-state image pickup device is shown in FIGS. 1(a) and 1(b). FIG. 1(a) is a plan view showing the solid-state image pickup device, and FIG. 1(b) is a cross sectional view taken along the lines I.sub.b -I.sub.b of FIG. 1(a). On the surface of a p-type semiconductor substrate 1, an island-shaped n-type impurity region 2 is formed. By the pn junction of the p-type semiconductor substrate 1 and the n-type impurity region 2, a photoelectric cell is formed. In addition, on the surface of the p-type semiconductor substrate 1, a transport channel 16 and an n-type impurity region 4 serving as a charge transfer channel are formed. The island-shaped n-type impurity region 2 is surrounded by a p-type p.sup.+ impurity region 3 and is isolated from the adjacent impurity region 2. On the impurity region 4 and the transport channel 16 formed on the semiconductor substrate 1, a charge transfer electrode 5 is provided through an insulating layer 6.
When a light is incident to the n-type impurity region 2, signal charges are produced on the pn junction surface between the p-type semiconductor substrate 1 and the n-type impurity region 2. In cases where the potential of the charge transfer electrode 5 represents L level, the signal charges 9 and 10 thus produced are accumulated or stored into the n-type impurity region 2 as shown in FIG. 1(c). In this case, the level 8 is a potential level of the p.sup.+ impurity region 3 for isolation between cells, the level 7 is a level of the transport channel 16 when the charge transfer electrode 5 is at L level, and the level 11 is a level of the n-type impurity region 2 which has been lowered by the accumulation of the signal charges 9 and 10.
When the charge transfer electrode 5 shifts to H level, the level 7 is raised to the level 15 as shown in FIG. 1(d). As a result, the signal charge 10 whose level is smaller than the level 15 is transported to the n-type impurity region 4 via the transport channel 16 and is accumulated thereinto. Thus, the level of the n-type impurity region 2 is raised to the level 12, so that the signal charge 9 remains therein.
Immediately after the charge transportation has been conducted, the n-type impurity region 2 is greatly reverse-biased by the potential difference between the level 12 of the n-type impurity region 2 and the level 8 of the n-type impurity region 3 for isolation between cells. Accordingly as signal charges are accumulated, the reverse bias voltage lowers. Accordingly, the width W of the depletion layer formed by the n-type impurity region 2 and the p.sup.+ impurity region 3 surrounding it becomes narrower as signal charges accumulated increase. As a result, the capacity of the junction of the n-type impurity region 2 and the p.sup.+ impurity region 3 becomes large. In general, the width W of the depletion layer is proportional to one-half power of the reverse bias voltage and depicts a characteristic curve as shown in FIG. 2. When the impurity concentration of the n-type impurity region 2 is high, charges 9 remaining in the n-type impurity region 2 after transportation are increased. As a result, there occurs to much extent the well known thermal charge emission wherein charges gradually move to the n-type impurity region 4 serving as the transfer channel, giving rise to afterimage in the solid-state image pickup device.
For this reason, the n-type impurity region 2 is formed so that the impurity concentration thereof is sufficiently lower than that of the p.sup.+ impurity region 3 serving as the layer for isolation between cells. Thus, the depletion layer is formed principally within the n-type impurity region 2. Reference numerals 13 and 14 in FIG. 1(a) represent a charge accumulated condition and the edge of the depletion layer within the n-type impurity region 2 immediately after transportation, respectively.
On the other hand, when the impurity concentration of the n-type impurity region 2 is low, the width W of the depletion layer becomes broader. Thus, the junction capacity is lowered, with the result that the charge accumulation capacity becomes small. Accordingly, it is difficult to ensure a necessary accumulation capacity to lower the impurity concentration of the n-type impurity region 2 to much extent. As a result, there was nothing to do but tolerate that afterimage occurred to some extent.